A system enabling a small focused footprint of small dry ice particles to be directed onto a surface to be cleaned by a dry ice blasting stream at a low rate of gas flow.
Dry ice blasting involves the generation and discharge of a blasting stream consisting of dry ice particles and a carrier stream of a gas under pressure. The gas is usually air, although other gases such as nitrogen, carbon dioxide, and argon may be used instead.
The particle sizes of dry ice employed for various cleaning tasks extend from large pellets to small particles such as shavings. The selection depends in large part on the desired energy of impact, and of the ability of the surface being cleaned to resist the impact of the particles. A delicate article can be cleaned with very small particles, but could be destroyed by larger ones.
Dry ice blasting with small particles is known, but for reasons to be described, they have never before been utilized to provide a very closely held, tightly focused, small diameter impact pattern. This in large part is due to the wide range of sizes customarily produced in a single sample of particles, plug the nozzle, bringing the process to a halt. Accordingly, when cleaning with small particles, existing nozzles and apparatus appropriate to heavy cleaning tasks were used, often inefficiently, and never with small patterns and small nozzles.
These prior devices often were undesirable neighbors operating in the pressure range between about 50 to 300 psi, and with volumes at 100 psi of perhaps 100-250 cfm. These are very loud systems, often 100-130 dbA, and very power hungry. At 100 psi a typical dry ice blasting machine that consumes at least 150 cfm requires a 40-50 horsepower compressor.
Such apparatus is poorly suited for precision cleaning of delicate areas, such as in mold cavities with restricted access. The full advantages of dry ice blasting have not been available for such applications until now. In fact, it was possible to clean these areas only with great inconvenience.
It is an object of this invention to provide a useful, finely focused beam of a blasting stream with small particles, with an order of magnitude less of air and remarkably reduced noise. For example, a nozzle operating at 75 psi can consume as little as 4 cfm, from a one horsepower compressor. The noise can readily be reduced to about 80-90 dbA. The ability to focus a useful beam of this stream on a small delicate area opens an entirely new field of usage for the basic dry ice blasting process.
A low flow nozzle receives and discharges a stream of dry ice particles in a carrier stream of gas. The gas is derived from a pressure source at a regulated rate. The dry ice is generated by a particle generator which produces a supply of granules substantially lacking in sizes above an upper limit, and at a rate respective to a desired mass ratio of air to dry ice particles.
The blasting stream passes through a discharge orifice whose cross section dimensions are related to the nominal size of the particles so that plugging the orifice is unlikely.
According to the preferred embodiment of the invention, the nominal size of the particles is no greater than about 0.035 inches in diameter, and their weight less than about 0.00016 ounces. Also, the mass ratio of air to dry ice is preferably between about 6:1 and about 1:1, and the rate of air flow is about 2-40 cfm. These are optimum relationships to produce a small footprint, perhaps as small as 0.125 inches diameter.